1. Field of the Invention
The present invention relates to a printing head for use with a color ink-jet printer of an interlace drive system or the like and an image data printing method using such printing head.
2. Description of Prior Art
In a printing apparatus for printing image data by moving a printing head relative to a printable medium such as a sheet of paper, as important factors which affect a printing quality, there are enumerated a transport accuracy with which the printable medium is transported in the sub-scanning direction and a printing position accuracy of printing head. For achieving this transport accuracy and the printing position accuracy, there is enumerated an interlace drive system.
The interlace drive system includes a printing head comprising a plurality of nozzle (ink-jet section) arrays spaced apart in the sub-scanning direction by an interval of an integer multiple of a printing resolution in which a nozzle space P and a nozzle number q (not the number of existing nozzles but the number of nozzles in actual use) are mutually prime, and prints adjacent lines by different nozzles while a printable medium is being fed at a constant pitch in the sub-scanning direction.
According to the printing apparatus of interlace drive system, since a fluctuation in the interval in which the nozzles are spaced apart does not remarkably affect the printing quality and the transport amount of the printable medium is always constant, a fluctuation in the transport accuracy is small, and hence a high printing quality can be provided. Furthermore, since the interval of the nozzles which are disposed is wide, even a printing head of high resolution can be manufactured with ease.
However, of the printing apparatus of interlace drive system, when, in particular, a printer of a type such as a color ink-jet printer for printing an image by ink supplying is used to print binary pseudo gray-scale (middle-tone) image data provided by a dot-distribution type dither matrix, there may occur the following problems.
For example, the problem caused by the dot-distribution dither matrix in which the nozzle number=63 (63 in actual practice out of the existing nozzle number=64), the nozzle space S=2 and gradation level of gray-scale image lies in a range of from 1 to 64 as shown in FIG. 14A will be described below. This dither matrix is formed by repeatedly disposing a fundamental layout pattern of threshold values shown in FIG. 14B.
When an image in which all pixels comprise C (cyan)=16, M (magenta)=0 and Y (yellow)=32 is binary-processed as a multiple-value gray-scale image, the position at which C is ON (position at which ink of C is jetted) is located on the odd-numbered line Od in the lines of the dither matrix as shown in FIG. 15.
The position at which Y is ON (position at which ink of Y is jetted) is located at the same position at which C is ON and on the even-numbered line Ev as shown in FIG. 16.
When the thus binary-processed pseudo-gray-scale image is printed by the interlace drive system, initially, only odd-numbered lines Od are printed by using 63 nozzles per each color in the first main scan as shown in FIG. 17. As is clear from FIGS. 15 and 16, C and Y are printed on the odd-numbered lines Od, and hence inks of C and Y of the same ratio are jetted.
The printing head is relatively moved (sub-scanning) by an amount of 32 nozzles in the sub-scanning direction, and the printing apparatus prints image data by the second main scanning. Only the even-numbered line Ev is printed by the second main scanning. Since only Y is ON as is clear from FIGS. 15 and 16, only Y is printed.
The printing head is relatively moved by an amount of 32 nozzles in the sub-scanning direction, and the printing apparatus prints image data by the third main scanning. Since only the odd-numbered line Od is printed by the third main scanning, inks of C and Y are jetted similarly to the first main scanning.
In the next fourth main scanning, only inks of Y are jetted similarly to the second main scanning.
In this way, an image of the same coloring with C (cyan)=16, M (magenta)=0 and Y (yellow)=32 is printed with a resolution twice as high as the interval of the nozzles.
However, since the printing apparatus is of the type of the interlace drive system, as shown in FIG. 17, inks of C and Y are jetted first and inks of Y are jetted later in a print overlapping area R1 of the first main scanning and the second main scanning. In a print overlapping area R2 of the second main scanning and the third main scanning, inks of Y are jetted first and inks of C and Y are jetted later. In a print overlapping area R3 of the third main scanning and the fourth main scanning, inks of C and Y are jetted first and inks of Y are jetted later similarly to the overlapping area R1.
Since the overlapping areas R1 to R3 are the same coloring areas, inks of C and Y of the same amount are jetted. However, the orders in which inks are jetted are different in the adjacent overlapping areas R1 to R3. Inks jetted on the even-numbered line Ev and inks jetted on the odd-numbered line Od are not independent perfectly but overlap with each other in the portion in which their lines are adjacent to each other. If the order of overlapped inks is reversed in this overlapping portion due to the error of the order in the ink-jet, then a small difference of features between the inks such as a degree of blur causes a small difference to occur from a standpoint of color. As a consequence, although the overlapping areas R1, R2, R3 are the same coloring areas as the gray-scale image, they are reproduced as striped patterns.
In the above example, there has been described so far the interlace drive system in which the printing head scans alternately odd 2s+1-th line and even 2 (s+1)-th line (s is an integer not less than 0), i.e., at the unit of 2 lines. A similar phenomenon occurred in the interlace drive system in which odd-numbered line and even-numbered line are alternately scanned at the unit of not less than 3 lines. FIG. 18, for example, shows an example of another interlace drive system in which printing heads 200 are spaced apart at the unit of 3 lines by 2 nozzles between the nozzles and printing 3s+1-th line, 3s+2-th line and 3(s+1)-th line (s is an integer not less than 0) by moving relatively the printing head 200 by an amount of substantially 1/3 of the length of the printing head 200 in the sub-scanning direction. Although the printing heads 200 are provided for respective colors, only one printing head 200 is shown in order to facilitate the understanding. In FIG. 18, an reference numerals arrayed in the lateral direction denote numbers indicating positions of the printing head 200 in the sub-scanning direction, respectively. With respect to reference numerals described along the printing head 200, "1" denotes 3s+1-th line, "2" denotes 3s+2-th line and "3" denotes 3(s+1)-th line, respectively.
Also in this case, in the first overlapping area R11 (printing based on the positions 1, 2, 3 in the sub-scanning direction), the printing head 200 prints lines in the sequential order of 3(s+1)-th line, 3s+2-th line, 3s+1-th line (i.e., in the sequential order of "321"). However, in the next overlapping area R12 (printing based on the positions 2, 3, 4 in the sub-scanning direction), the printing head 200 prints lines in the sequential order of 3s+2-th line, 3s+1-th line, 3(s+1)-th line (i.e., in the sequential order of "213"). In the next overlapping area R13 (printing based on the positions 3, 4, 5 in the sub-scanning direction), the printing head 200 prints lines in the sequential order of 3s+1-th line, 3(s+1)-th line, 3s+2-th line (i.e., in the sequential order of "132"). Further, in the next overlapping area R14 (printing based on the positions 4, 5, 6 in the sub-scanning direction), the printing head 200 prints lines in the sequential order of 3(s+1)-th line, 3s+2-th line and 3s+1-th line (i.e., in the sequential order of "321").
Accordingly, if the size of the fundamental layout pattern of the threshold values in the sub-scanning direction is a multiple of 3+1 and printed lines are deviated on a specific lines within 3s+1-th line, 3s+2-th line and 3(s+1)-th line at a in certain concentration, then the orders in which colors are overlapping become different in the areas of the same color, thereby resulting in striped patterns.